This is the sort of stuff that keeps me up nights – so, here's one for this week, asked by my 8 year old son yesterday – why is it called a flea market? Here's an explanation from the word-detective.com web site (fun site…all sorts of trivia)The itch to shop.Dear Word Detective: Why are flea markets called "flea" markets? How
did they get their name and why? — Danny Sayer, via the Internet.

In any case, "flea market" first appeared in English in the 1920's and is most likely a simple translation of the French market's name.